1. Field of the Invention
The present invention relates to a process for continuous extraction of polyamides, especially nylon 6, by treating polyamide particles in a vertical extraction column with recirculating aqueous .epsilon.-caprolactam solution.
Nylon 6 (polycaprolactam) is produced by polymerization of .epsilon.-caprolactam. The polycaprolactam obtained has a caprolactam monomer and oligomer content of, for example, from 8 to 11% by weight. Left in the polycaprolactam product, these low molecular weight constituents (in the following refered to as extractables) cause undesirable effects in further processing of the polymer product and must therefore be removed. Industrially, this is accomplished by continuous or batchwise extraction with hot water (DE-A-25 01 348, DE-A-27 32 328) and by distillative removal under reduced pressure (U.S. Pat. No. 4,376,680) or in superheated steam (EP 0 284 968 B1). These processes are all carried out with an eye to recovering and reusing the extractables for reasons of environmental protection and economy. For nylon 6, these processes leave residual extractables (methanol-extractables) consisting essentially of caprolactam oligomers which are sparingly soluble in water or involatile, especially dimers and cyclic oligomers.
2. Description of the Related Art
Various apparatus has been proposed for extracting low molecular constituents from polyamides. GB 12 97 606 discloses an extraction column that is divided into at least two zones, the extractant being recirculated within each zone in countercurrent to the flow of liquid by removal at the upper end of the zone and reintroduction at the lower end of the zone. Similar apparatus is described, for example, in CZ 253 019, FR 15 18 775, DD 206 999 and DE-A-17 70 097.
It is known that caprolactam monomer acts as a solublizer for caprolactam oligomer in the extraction of nylon 6. This is why, for example according to JP-A-47 026438, the nylon 6 chips are pretreated with a solution of from 15 to 90%, preferably of from 40 to 70%, of .epsilon.-caprolactam to remove the water-solubles. In DD 289 471, the chips are treated in countercurrent at above 60.degree. C. with from 1 to 40% of caprolactam in the wash water (the percentages are each by weight). DE-A-43 24 616 discloses a process for extracting nylon 6 to obtain products having a very low residual level of dimeric .epsilon.-caprolactam. Here, a first stage, which features an extraction with from 41 to 80% caprolactam solution at from 80 to 120.degree. C., is followed by one or more postextraction stages, either with water at high temperature or under reduced pressure. In JP-A-48 002 233 polycaprolactam is purified by admixing the molten polymer with from 5 to 30% strength caprolactam solution and then purifying the resulting dispersion at from 80 to 120.degree. C. in an extraction column. In JP-A-53 071 196, polyamide is initially extracted with a hot aqueous medium and then purified at from 10 to 50.degree. C. below the melting point of the polyamide in an inert gas stream, the hot aqueous medium comprising, for example, water at from 80 to 130.degree. C. with an .epsilon.-caprolactam content of less than 50% by weight. JP A-45 025 519 discloses a multistage extraction process wherein the polyamide chips are extracted with from 5 to 50% strength aqueous caprolactam solution at from 70 to 120.degree. C. in the first stage and with from 0.1 to 5% strength aqueous caprolactam solution at from 70 to 120.degree. C. in the second stage. JP-A-51 149 397 describes an extraction with an aqueous 60% strength by weight .epsilon.-caprolactam solution at from 80 to 120.degree. C. for from 3 to 8 hours in the first stage and an extraction with caprolactam-free water, which is preferably O.sub.2 -free or comprises small amounts of a reducing agent, in the last stage. These processes too are preferably carried out with recovery and reuse of the extractables for reasons of environmental protection and economy. Accordingly, JP-A-60 166 324 discloses a continuous nylon 6 extractor wherein the chips are extracted with water in countercurrent by recirculating the bulk of the extraction liquid with addition of .epsilon.-caprolactam. The extractant is pumped off through an aspirator, admixed with caprolactam and returned into the apparatus via a distributor located at the same level as the aspirator. The pumped-off extract has a caprolactam content of only 13.5%, i.e., less than 20%.
In DE-A-195 05 150 the caprolactam oligomer is removed from polyamide chips by treatment with pure caprolactam as extractant at from 60 to 150.degree. C. However, this method has the disadvantage that adherent caprolactam may lead to stickiness of the chips in subsequent operations. Moreover, at these temperatures, the chips would also dissolve in caprolactam to some extent.
Using water or water vapor as extractant for the polyamide chips it is very difficult to achieve the present-day requirement of residual extractables contents &lt;0.5%. The extract obtained will typically be a solution having an extractables content of from 5 to 15%, similar to what is obtained using caprolactom-comprising extractants. The extract may additionally include inorganics such as titanium dioxide, silicon dioxide and manganese oxide, typically added to the polyamide for stabilization or delustering. Existing processes have in common that either the residual extractables content of the chips is too high or that the aqueous extract has to be highly concentrated in order that the caprolactam monomer and caprolactam oligomer may be recycled into the polymerization. Oligomer and inorganics may separate out during the concentrating, which also has appreciable energy requirements.